This invention relates to laminate structures and is more particularly directed to improving the resistance thereof to the initiation and propagation of fatigue cracking and to subsequent fracture.
Structural materials for military weapons and vehicles must possess a high level of resistance to fracture as well as the ability to sustain considerable impact damage without incurring the type of rapid fatigue crack propagation which leads to catastrophic failure. Accordingly, it has been proposed that the critical portions of military items be fabricated from composite laminates since the layered structure thereof increases the initial amount of impact energy which can be absorbed prior to the initiation of fatigue cracking while the interface surfaces of the individual layers act to retard the subsequent propagation of such cracking during cyclic loading. However, various attempts to develop laminates as a basic structural material for damage-prone military equipment have generally failed to gain wide acceptance since the resulting increase in crack resistance and the corresponding decrease in the rate of crack propagation have not been of sufficient extent to outweigh the greater cost generated by the increased complexity of manufacture of both the material and the end item fabricated therefrom.